U.S. patent number 7,311,775 [Application Number 11/196,480] was granted by the patent office on 2007-12-25 for method for heat-treating silicon wafer and silicon wafer.
This patent grant is currently assigned to Sumco Corporation. Invention is credited to Tatsumi Kusaba, Yoshihisa Nonogaki, Hidehiko Okuda.
United States Patent |
7,311,775 |
Kusaba , et al. |
December 25, 2007 |
Method for heat-treating silicon wafer and silicon wafer
Abstract
This method for heat-treating a silicon wafer includes: a step
of subjecting a silicon wafer to a high-temperature heat treatment
in an ambient gas atmosphere of hydrogen gas, argon gas or a
mixture thereof; and a step of lowering a temperature at a rate of
2.degree. C./min or less in a nitrogen-gas-containing ambient
atmosphere in a portion or all of a process of lowering a
temperature to a wafer removal temperature following said
high-temperature heat treatment. This silicon wafer has a
defect-free layer which is formed by a high-temperature heat
treatment and is included in a surface thereof, wherein an average
iron concentration in said surface is 1.times.10.sup.10
atoms/cm.sup.3 or less.
Inventors: |
Kusaba; Tatsumi (Takeo,
JP), Okuda; Hidehiko (Imari, JP), Nonogaki;
Yoshihisa (Imari, JP) |
Assignee: |
Sumco Corporation (Tokyo,
JP)
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Family
ID: |
34998659 |
Appl.
No.: |
11/196,480 |
Filed: |
August 4, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060027161 A1 |
Feb 9, 2006 |
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Foreign Application Priority Data
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Feb 9, 2004 [JP] |
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P2004-032603 |
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Current U.S.
Class: |
117/86; 117/84;
117/89 |
Current CPC
Class: |
C30B
29/06 (20130101); C30B 33/02 (20130101) |
Current International
Class: |
C30B
25/12 (20060101); C30B 25/14 (20060101) |
Field of
Search: |
;117/84,86,89 |
Foreign Patent Documents
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5-18254 |
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Dec 1985 |
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JP |
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3080501 |
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Sep 1994 |
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JP |
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06-295913 |
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Oct 1994 |
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JP |
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10-144698 |
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May 1998 |
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JP |
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2000-058552 |
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Feb 2000 |
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JP |
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Primary Examiner: Hiteshew; Felisa
Attorney, Agent or Firm: Pillsbury Winthrop Shaw Pittman,
LLP
Claims
What is claimed is:
1. A method for heat-treating a silicon wafer, the method
comprising: subjecting a silicon wafer to a high-temperature heat
treatment in an ambient gas atmosphere of hydrogen gas, argon gas
or a mixture thereof; and lowering a temperature at a rate of
2.degree. C./min or less in a nitrogen-gas-containing ambient
atmosphere in a portion or all of a process of lowering a
temperature to a wafer removal temperature following said
high-temperature heat treatment.
2. The method for heat-treating a silicon wafer according to claim
1, wherein said lowering a temperature at a rate of 2.degree.
C./min or less in a nitrogen-gas-containing ambient atmosphere is
carried out in a temperature range of 700.degree. C. or less during
said process of lowering a temperature.
3. The method for heat-treating a silicon wafer according to claim
2, wherein said high-temperature heat treatment is a heat treatment
in a temperature range of 900 to 1350.degree. C. for one hour or
more.
4. The method for heat-treating a silicon wafer according to claim
1, wherein said high-temperature heat treatment is a heat treatment
in a temperature range of 900 to 1350.degree. C. for one hour or
more.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to, but does not claim priority from,
Japanese Patent Application No. 2004-032603, filed on Feb. 9, 2004,
the contents of which are incorporated herein in their entirety by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for heat-treating silicon
wafers and to silicon wafers.
2. Background Art
A surface layer portion of a silicon wafer sometimes contains
oxygen precipitates and crystal defects having a regular octahedral
structure known as crystal originated particles (COP) which are
introduced into a crystal during pulling of a silicon
single-crystal ingot. When present in the surface layer portion of
the wafer where devices are fabricated, these defects cause a
deterioration in the gate oxide integrity and other electrical
characteristics.
Therefore, in order to improve the gate oxide integrity and other
electrical characteristics of the silicon wafer, it is essential
for the surface layer portion of the wafer where devices are
fabricated to be made a defect-free layer. Numerous reports (e.g.,
Patent Documents 1 to 5) indicate that in order to form a
defect-free layer in a surface layer portion of a silicon wafer and
thus improve the gate oxide integrity, it is effective to subject
the silicon wafer to a high-temperature heat treatment in an
ambient atmosphere of hydrogen, argon, or a gas mixture thereof for
several hours.
As disclosed in Patent Document 3 and Patent Document 4,
temperature processes in such high-temperature heat treatment
typically involve a heat-up process in which a temperature is
raised from a wafer loading temperature to 1000.degree. C. at a
rate of about 10.degree. C./min and is raised from 1000 to
1200.degree. C. at a rate of 3.degree. C./min or less, a heat
treatment which is carried out at about 1200.degree. C. for one
hour or more, and a ramp-down process in which the temperature is
lowered from 1200.degree. C. to about 1000.degree. C. at a rate of
3.degree. C./min and is lowered at 1000.degree. C. or below at a
rate of 10.degree. C./min.
In the ramp-down process, the reason for setting a ramp-down rate
to 3.degree. C./min within a temperature range of 1200 to
1000.degree. C. is to allow oxygen precipitates to form in the
wafer by lowering the ramp-down rate in this temperature range. The
reason for setting the ramp-down rate to 10.degree. C./min within a
temperature range of 1000.degree. C. or below is to increase
throughput and reduce production costs by raising the ramp-down
rate.
In a high-temperature heat treatment in a hydrogen gas-containing
ambient atmosphere, it is well-known that a reducing action of the
hydrogen gas etches surfaces of a quartz reaction tube and a wafer
boat (which is made of quartz, silicon or SiC), thereby, during the
heat treatment, a silicon wafer surface is contaminated with
impurities present in a parent material, especially heavy metals
such as iron, copper and nickel.
There is no effective means for removing heavy metal impurities
once they have diffused to the wafer surface as a result of the
high-temperature heat treatment. Hence, there exists a need for a
method for heat-treating a silicon wafer which does not give rise
to heavy metal contamination. Recently, a high-temperature heat
treatment in an ambient atmosphere of argon gas which does not have
a reducing effect has been regarded as promising.
However, in our own experiments, we have found that iron
concentrations (iron contamination) in a silicon wafer surface were
higher than those prior to a high-temperature heat treatment, not
only in silicon wafers subjected to a high-temperature heat
treatment in a hydrogen-containing ambient atmosphere, but also
even in silicon wafers subjected to a high-temperature heat
treatment in an ambient atmosphere of argon gas or an ambient
atmosphere of argon gas containing a small amount of hydrogen gas
mixed therein.
Patent Document 1: Japanese Examined Patent Application, Second
Publication No. H05-18254
Patent Document 2: Japanese Patent No. 3080501
Patent Document 3: Japanese Patent Application, First Publication
No. H06-295913
Patent Document 4: Japanese Patent Application, First Publication
No. H10-144698
Patent Document 5: Japanese Patent Application, First Publication
No. 2000-58552
SUMMARY OF THE INVENTION
In light of the above problems, it is an object of the present
invention to provide silicon wafers having a defect-free layer in a
surface layer portion of the silicon wafer and having a low heavy
metal contamination (iron contamination) in a wafer surface.
Another object of the present invention is to provide a method for
heat-treating a silicon wafer for obtaining such silicon
wafers.
As a result of extensive investigations conducted in order to
achieve these objects, we have discovered that iron contamination
in the wafer surface can be suppressed by keeping a rate for
lowering a temperature low in a specific ambient gas atmosphere
during a step of lowering a temperature following a
high-temperature heat treatment.
Accordingly, a method for heat-treating a silicon wafer of the
present invention includes: a step of subjecting a silicon wafer to
a high-temperature heat treatment in an ambient gas atmosphere of
hydrogen gas, argon gas or a mixture thereof; and a step of
lowering a temperature at a rate of 2.degree. C./min or less in a
nitrogen-gas-containing ambient atmosphere in a portion or all of a
process of lowering a temperature to a wafer removal temperature
following said high-temperature heat treatment.
According to the present invention, a sufficient defect-free layer
can be achieved in the surface layer portion of the wafer, and iron
contamination in the wafer surface can be suppressed. Although the
reason for the decrease in iron contamination is not yet
understood, in the case in which a temperature is lowered at a rate
greater than 2.degree. C./min in a nitrogen-gas-containing ambient
atmosphere, this desirable effect is not obtained; nor is this
effect obtained in the case in which a temperature is lowered at a
rate of 2.degree. C./min or less in an ambient gas atmosphere other
than a nitrogen-gas-containing ambient atmosphere. We have found
that this effect occurs only when both conditions of the gas
ambient atmosphere and the rate for lowering a temperature are
completed.
It is desirable for the ambient gas atmosphere used in the
high-temperature heat treatment of the present invention to be 100%
hydrogen gas, 100% argon gas, or an ambient mixed gas atmosphere
composed of argon gas containing a small amount of hydrogen gas
mixed therein. The ambient gas atmosphere during lowering a
temperature is preferably 100% nitrogen gas, although it may
instead be an ambient gas atmosphere composed of nitrogen gas
containing a small amount of a non-oxidizing gas (argon gas,
hydrogen gas and the like) mixed therein. The rate for lowering a
temperature is more preferably 1.degree. C./min or less. However,
if the rate for lowering a temperature is too slow, the throughput
will become very long, which is undesirable in terms of
productivity. From this standpoint, it is preferable to ensure a
rate for lowering a temperature of 0.5.degree. C./min or more.
In the method for heat-treating a silicon wafer of the present
invention, the lowering a temperature at a rate of 2.degree. C./min
or less in a nitrogen-gas-containing ambient atmosphere may be
carried out in a temperature range of 700.degree. C. or less during
the process of lowering a temperature. This enables iron
contamination of the wafer surface to be further suppressed.
A sufficient effect of suppressing iron contamination is achieved
in the wafer surface by carrying out all the process of lowering a
temperature at a rate of 2.degree. C./min or less in a
nitrogen-gas-containing ambient atmosphere. However, in the case in
which a gas replacement to a nitrogen-gas-containing ambient
atmosphere is carried out in a temperature range of more than
700.degree. C., nitrides are formed in the wafer surface, making it
necessary to add a new step to remove the nitrides. Hence, the gas
replacement to the nitrogen-gas-containing ambient atmosphere is
preferably conducted in a temperature range of 700.degree. C. or
less, and the gas replacement in a temperature range of 650.degree.
C. or less is especially preferred. Also, because the effect of
suppressing iron contamination cannot be achieved even if the gas
replacement to the nitrogen-gas-containing ambient atmosphere is
administered in a low temperature range of less than 500.degree.
C., the gas replacement must be carried out in a temperature range
of 500.degree. C. or more. Accordingly, in order to suppress both
nitride formation and iron contamination, it is advantageous for
lowering a temperature to be carried out in a temperature range
from 650.degree. C. or less to 500.degree. C. or more and in a
nitrogen-gas-containing ambient atmosphere.
In the practice of the present invention, the high-temperature heat
treatment carried out on the silicon wafer may be a heat treatment
in a temperature range of 900 to 1350.degree. C. for one hour or
more. At less than 900.degree. C., defects such as COPs and oxygen
precipitates in the surface layer portion in the wafer cannot be
reduced sufficiently, whereas at more than 1350.degree. C., defects
such as slip dislocations may be newly introduced in the wafer.
Also, at a heat treatment time of less than one hour, defects such
as COPs and oxygen precipitates in the surface layer portion in the
wafer cannot be reduced sufficiently. Lengthening the heat
treatment time is effective for eliminating defects such as COPs,
but this leads to greater production costs due to an increase in a
cycle time and an increased burden (wear and tear) on an equipment.
Therefore, it is desirable to limit the heat treatment time to less
than 12 hours at a maximum.
The silicon wafer of the present invention has a defect-free layer
which is formed by a high-temperature heat treatment and is
included in a surface thereof, wherein an average iron
concentration in the surface is 1.times.10.sup.10 atoms/cm.sup.3 or
less.
According to the present invention, by subjecting a silicon wafer
to a high-temperature heat treatment in an ambient gas atmosphere
of hydrogen gas, argon gas or a gas mixture thereof, and then
lowering a temperature at a rate of 2.degree. C./min or less in a
nitrogen-gas-containing ambient atmosphere in a portion or all of a
process of lowering a temperature to a wafer removal temperature, a
sufficiently defect-free layer can be achieved in the surface layer
portion of the wafer and iron contamination can be adequately
suppressed in the wafer surface. Moreover, although the wafer of
the present invention is an annealed wafer heat-treated at a high
temperature, it is of a high quality characterized by an average
iron concentration in the silicon wafer surface of
1.times.10.sup.10 atoms/cm.sup.3 or less, enabling remarkable
improvements to be made in device characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing heat treatment temperature conditions in
examples of the present invention and in comparative examples.
PREFERRED EMBODIMENTS
Embodiments of a method for heat-treating a silicon wafer of the
present invention is described below with reference to FIG. 1.
Silicon wafers are prepared by known operations such as slicing,
lapping, grinding, polishing and cleaning treatment from silicon
single-crystal ingots grown by the Czochralski method (CZ
method).
The silicon wafers are loaded on a thermal processing boat, after
which the silicon wafers are introduced into a thermal processing
furnace having an argon ambient atmosphere. As shown in FIG. 1, in
a heat-up step S1, a temperature of the silicon wafers is raised at
a given heat-up rate to a temperature of a high-temperature heat
treatment. When the heat treatment is carried out in a hydrogen gas
ambient atmosphere or a mixed gas ambient atmosphere of hydrogen
gas and argon gas, it is necessary to eliminate a danger of an
explosion due to contact between an outside atmosphere and the
hydrogen gas. To this end, the thermal processing boat is loaded
into the thermal processing furnace while an interior of the
furnace is under a nitrogen gas ambient atmosphere, and then the
interior of the furnace is replaced with the hydrogen gas ambient
atmosphere or the mixed gas ambient atmosphere of hydrogen gas and
argon gas. Thereafter the heat-up is begun. The heat-up rate
employed in the heat-up step SI may be any suitable rate and may be
suitably selected according to various wafer conditions. Also, any
suitable thermal processing apparatus may be used to work the
present invention, such as a vertical thermal processing apparatus,
a horizontal thermal processing apparatus, or a thermal processing
apparatus for single wafers.
In a high-temperature heat treatment step S2, heat treatment is
carried out in a temperature range of 900 to 1350.degree. C. for
one hour or more. This results in the formation in the wafer
surface of a defect-free layer in which COPs and oxygen
precipitates are absent.
Next, in a lowering temperature step S3, the interior of the
thermal processing furnace is changed to a nitrogen gas ambient
atmosphere, and the temperature is lowered at a rate of 2.degree.
C./min or less. This enables iron contamination of the wafer
surface to be suppressed. Here, following completion of the
high-temperature heat treatment step S2, the interior of the
thermal processing furnace may be immediately flushed with a
nitrogen-gas-containing ambient atmosphere and a temperature may be
started to be lowered at a rate of 2.degree. C./min or less (line
A). However, in such a case, the throughput in the lowering
temperature step S3 as a whole becomes longer and nitride formation
occurs in the wafer surface. Therefore, following completion of the
high-temperature heat treatment step S2, it is desirable to
initially lower the temperature at a lowering rate of more than
2.degree. C./min, then change to a nitrogen gas ambient atmosphere
at a temperature range of 700.degree. C. or less and subsequently
start lowering a temperature at a rate of 2.degree. C./min or less
(line B). In this way, both nitride formation and iron
contamination can be suppressed.
EXAMPLES
The method for heat-treating a silicon wafer of the present
invention is illustrated in the following examples. Specifically,
heat treatment experiments were carried out in which relationship
between the ambient gas atmosphere and a lowering rate during
lowering a temperature in the heat treatment of the silicon wafers
was varied and iron concentration detected in the wafer surface was
measured. Those heat treatment conditions and results obtained are
described below.
The silicon wafers used in each experiment were all boron-doped
p-type silicon wafers which had a diameter of 200 mm and a crystal
plane orientation of (100) and had been cut from a single-crystal
silicon ingot grown by the Czochralski (CZ) method, and mirror
polished.
Common heat treatment conditions in the examples of the present
invention and comparative examples were as follows. The
above-described silicon wafers were loaded into a vertical thermal
processing boat, and then the boat was placed in a thermal
processing furnace. A temperature in the furnace was raised to
1000.degree. C. at a fixed heat-up rate of 10.degree. C./min, and
then the heat-up rate was changed to 1.degree. C./min. Once the
temperature had reached 1200.degree. C., the temperature was held
at that level and a heat treatment of the wafer was carried out for
one hour.
In Example 1 of the present invention, high-temperature heat
treatment at 1200.degree. C. in an argon gas ambient atmosphere was
carried out for one hour, and then an interior of the thermal
processing furnace was immediately changed to a 100% nitrogen gas
ambient atmosphere and the temperature was lowered to 500.degree.
C. at a lowering rate of 2.degree. C./min.
In Example 2 of the present invention, high-temperature heat
treatment at 1200.degree. C. in an argon gas ambient atmosphere was
carried out for one hour, and then the interior of the thermal
processing furnace was immediately changed to a 100% nitrogen gas
ambient atmosphere and the temperature was lowered to 500.degree.
C. at a lowering rate of 1.degree. C./min.
In Example 3 of the present invention, high-temperature heat
treatment at 1200.degree. C. in an argon gas ambient atmosphere was
carried out for one hour. Next, while retaining the argon gas
ambient atmosphere in the thermal processing furnace, the
temperature was lowered to 1000.degree. C. at a lowering rate of
3.degree. C./min, and subsequently lowered to 700.degree. C. at a
lowering rate of 10.degree. C./min. The interior of the furnace was
then changed to a 100% nitrogen gas ambient atmosphere and the
temperature was lowered to 500.degree. C. at a lowering rate of
2.degree. C./min.
In Example 4 of the present invention, high-temperature heat
treatment at 1200.degree. C. in an argon gas ambient atmosphere was
carried out for one hour. Next, while retaining the argon gas
ambient atmosphere in the thermal processing furnace, the
temperature was lowered to 1000.degree. C. at a lowering rate of
3.degree. C./min, and subsequently lowered to 700.degree. C. at a
lowering rate of 10.degree. C./min. The interior of the furnace was
then changed to a 100% nitrogen gas ambient atmosphere and the
temperature was lowered to 500.degree. C. at a lowering rate of
1.degree. C./min.
In Comparative Example 1, high-temperature heat treatment at
1200.degree. C. in an argon gas ambient atmosphere was carried out
for one hour. While retaining the argon gas ambient atmosphere in
the thermal processing furnace, the temperature was then lowered to
500.degree. C. at a lowering rate of 3.degree. C./min.
In Comparative Example 2, high-temperature heat treatment at
1200.degree. C. in an argon gas ambient atmosphere was carried out
for one hour. While retaining the argon gas ambient atmosphere in
the thermal processing furnace, the temperature was then lowered to
500.degree. C. at a lowering rate of 1.degree. C./min.
In Comparative Example 3, high-temperature heat treatment at
1200.degree. C. in an argon gas ambient atmosphere was carried out
for one hour. And then, the interior of the thermal processing
furnace was immediately changed to a 100% nitrogen gas ambient
atmosphere and the temperature was lowered to 500.degree. C. at a
lowering rate of 3.degree. C./min.
In Comparative Example 4, high-temperature heat treatment at
1200.degree. C. in an argon gas ambient atmosphere was carried out
for one hour. While retaining the argon gas ambient atmosphere in
the thermal processing furnace, the temperature was then lowered to
1000.degree. C. at a lowering rate of 3.degree. C./min, and
subsequently lowered to 700.degree. C. at a lowering rate of
10.degree. C./min. The interior of the thermal processing furnace
was then changed to a 100% nitrogen gas ambient atmosphere and the
temperature was lowered to 500.degree. C. at a lowering rate of
3.degree. C./min.
In Comparative Example 5, high-temperature heat treatment at
1200.degree. C. in an argon gas ambient atmosphere was carried out
for one hour. While retaining the argon gas ambient atmosphere in
the thermal processing furnace, the temperature was then lowered to
1000.degree. C. at a lowering rate of 3.degree. C./min, then
lowered to 700.degree. C. at a lowering rate of 10.degree. C./min,
and finally lowered to 500.degree. C. at a lowering rate of
1.degree. C./min.
Each of the silicon wafers obtained in above Examples 1 to 4 of the
present invention and each of the silicon wafers obtained in
Comparative Examples 1 to 5 was subjected to a measurement of an
in-plane average concentration of iron in the wafer surface using
surface photo voltage (SPV) spectroscopy. In addition, the surface
of each wafer was examined by x-ray photoelectron spectroscopy
(XPS) for evidence of nitride formation thereon. The results are
given in Table 1.
TABLE-US-00001 TABLE 1 Ambient Fe Presence Temperature gas Lowering
concentration of range atmosphere rate (atoms/cm.sup.3) nitrides
Example 1 1200.degree. C. to 500.degree. C. nitrogen gas 2.degree.
C./min <1 .times. 10.sup.10 yes Example 2 1200.degree. C. to
500.degree. C. nitrogen gas 1.degree. C./min <1 .times.
10.sup.10 yes Example 3 1200.degree. C. to 700.degree. C. argon gas
2.degree. C./min <1 .times. 10.sup.10 no 700.degree. C. to
500.degree. C. nitrogen gas Example 4 1200.degree. C. to
700.degree. C. argon gas 1.degree. C./min <1 .times. 10.sup.10
no 700.degree. C. to 500.degree. C. nitrogen gas Comparative
1200.degree. C. to 500.degree. C. argon gas 3.degree. C./min >1
.times. 10.sup.10 no Example 1 Comparative 1200.degree. C. to
500.degree. C. argon gas 1.degree. C./min >1 .times. 10.sup.11
no Example 2 Comparative 1200.degree. C. to 500.degree. C. nitrogen
gas 3.degree. C./min >1 .times. 10.sup.11 yes Example 3
Comparative 1200.degree. C. to 700.degree. C. argon gas 3.degree.
C./min >5 .times. 10.sup.10 no Example 4 700.degree. C. to
500.degree. C. nitrogen gas Comparative 1200.degree. C. to
700.degree. C. argon gas 1.degree. C./min >5 .times. 10.sup.10
no Example 5 700.degree. C. to 500.degree. C. argon gas
As is apparent from Table 1, in Examples 1 to 4 of the present
invention, the average iron concentration detected in the surface
of the silicon wafers was 1.times.10.sup.10 atoms/cm.sup.3 or less
in each case, indicating a satisfactory effect of reducing iron
contamination. In particular, in the wafers obtained in Examples 3
and 4 of the present invention in which a replacement of nitrogen
gas was carried out at 700.degree. C. or less, the wafers were
found to be of a high quality with no observable nitride formation
in the wafer surface. On the other hand, the wafers obtained in
Comparative Examples 1 to 5 exhibited high levels of iron
contamination with 5.times.10.sup.10 atoms/cm.sup.3 or more irons
detected in each wafer.
Each of the silicon wafers obtained in Examples 1 to 4 of the
present invention and in Comparative Examples 1 to 5 was cleaved
and a cleavage face was subjected to Wright etching treatment, then
examined under an optical microscope to determine a thickness of a
defect-free layer formed in a surface layer portion of the wafer. A
defect-free layer of 10 .mu.m or more was found to have been formed
in the surface layer portion in each wafer.
Here, although high-temperature heat treatment was carried out in
an argon gas ambient atmosphere in each of the examples of the
present invention and in the comparative examples, the method of
the present invention is not limited to the use of an argon gas
ambient atmosphere in this step. Substantially similar results were
obtained when high temperature heat treatment was carried out in
hydrogen gas or in argon gas containing a small amount of hydrogen
(10%).
Since the silicon wafer of the present invention is heat-treated at
high temperature, the defect-free layer is formed in the surface
thereof. Accordingly, the wafer has an excellent gate oxide
integrity. Moreover, because the silicon wafer of the present
invention has a very low level of iron contamination in the surface
thereof, it functions effectively as a wafer in which defects in
device characteristics do not arise.
Preferred embodiments of the present invention have been described
above, although these embodiments are to be considered in all
respects as illustrative and not limitative. Those skilled in the
art will appreciate that various additions, omissions,
substitutions and other modifications are possible without
departing from the spirit and scope of the present invention as
disclosed in the accompanying claims.
* * * * *